Fundamentals of fish ecological integrity and their relation to the extended serial discontinuity concept

Current ecological theories and concepts describe running waters as four-dimensional systems, their longitudinal, lateral and vertical linkages, interactions and exchange processes varying over time and over different scales. According to Ward & Stanford's (1983) extended serial discontinuity concept (ESDC), along a three-reach river, model the relative strength of the longitudinal pathways is highest in the constrained headwaters, vertical interactions reach their maximum in the braided middle course and lateral connectivity plays the major role in alluvial floodplain rivers. The present paper examines the general tenets of the ESDC from a fundamental fish ecology perspective. Specifically, it focuses on the degree to which the spatial/temporal connectivity requirements of fish communities or key fish species along a schematic longitudinal river course are compatible with the underlying principles of the ESDC and whether these requirements provide basic criteria for assessing the ecological integrity of running waters. From the fish ecological perspective, the examples provided here demonstrate a principle agreement with the concept of the four-dimensional nature of running waters, whose key functional and structural elements are spatial/temporal fluvial dynamics, disturbances, connectivity, succession and ecotones. Fish are particularly useful indicators of the temporally variable connectivities on the full range of scales at the three spatial dimensions. As far as the habitat requirements of typical species/developmental stages are concerned, two major modifications of the ESDC are needed. In constrained headwaters, in addition to longitudinal connectivity, the crucial importance of the vertical pathway river/bed sediments for reproduction must be emphasized. Also, in addition to lateral connectivity, longitudinal connectivity remains a vital basis for potamal fish communities in alluvial braided and meandering zones.